fmd-math 0.1.2

//! The parse tree: [`Node`] and its supporting vocabulary.
//!
//! Every node carries its **byte span** into the source string — span
//! provenance is structural (the G0-3 ratification's §11.3 requirement), not
//! an afterthought: the span map that downstream consumers (`isolate`,
//! `tex_to_color_map`, `TransformMatchingTex`) use is derived from these
//! spans, so no node may ever be constructed without one.

use crate::atom::AtomClass;
use crate::style::Style;

/// A half-open byte range `[start, end)` into the source string.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct Span {
    /// Byte offset of the first byte of the construct.
    pub start: usize,
    /// Byte offset one past the last byte of the construct.
    pub end: usize,
}

impl Span {
    /// Construct a span. `start` and `end` are byte offsets; `end >= start`.
    #[must_use]
    pub const fn new(start: usize, end: usize) -> Self {
        Self { start, end }
    }

    /// The smallest span covering both `self` and `other`.
    #[must_use]
    pub fn union(self, other: Self) -> Self {
        Self {
            start: self.start.min(other.start),
            end: self.end.max(other.end),
        }
    }

    /// Length in bytes.
    #[must_use]
    pub const fn len(&self) -> usize {
        self.end.saturating_sub(self.start)
    }

    /// True when the span covers zero bytes.
    #[must_use]
    pub const fn is_empty(&self) -> bool {
        self.end <= self.start
    }
}

/// One parse-tree node: a kind plus the byte span it came from.
#[derive(Clone, Debug, PartialEq)]
pub struct Node {
    /// What the node is.
    pub kind: NodeKind,
    /// Where in the source string it came from.
    pub span: Span,
}

impl Node {
    /// Construct a node.
    #[must_use]
    pub const fn new(kind: NodeKind, span: Span) -> Self {
        Self { kind, span }
    }
}

/// A delimiter as named after `\left`, `\right`, or a `\big`-class command.
///
/// `ch` is `None` for the null delimiter `.` (as in `\right.`).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Delim {
    /// The delimiter character (already mapped to its math codepoint, e.g.
    /// `\langle` ⇒ `⟨`), or `None` for the null delimiter.
    pub ch: Option<char>,
    /// Source span of the delimiter token.
    pub span: Span,
}

/// The four fixed delimiter sizes of the `\big` family.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum DelimSize {
    /// `\big` class — 8.5 pt-per-10 pt nominal.
    Big,
    /// `\Big` class.
    BBig,
    /// `\bigg` class.
    Bigg,
    /// `\Bigg` class.
    BBigg,
}

/// How a big operator places its scripts.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
pub enum Limits {
    /// TeX's default: limits in display style for `\sum`-class operators,
    /// side scripts otherwise; `\int`-class operators default to side
    /// scripts in every style.
    #[default]
    Default,
    /// `\limits`: scripts above/below regardless of style.
    Limits,
    /// `\nolimits`: side scripts regardless of style.
    NoLimits,
}

/// The generalized-fraction flavors (rule 15's inputs).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct FracSpec {
    /// Draw the fraction bar.
    pub bar: bool,
    /// Delimiters wrapped around the whole fraction (`\binom`/`\choose`
    /// carry `( )`); `None` for plain fractions.
    pub delims: Option<(char, char)>,
    /// A forced layout style (`\dfrac` forces display, `\tfrac` text);
    /// `None` follows the ambient style.
    pub forced_style: Option<Style>,
}

/// The accent commands (both true accents and the wide over/under class).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum AccentKind {
    /// `\hat`
    Hat,
    /// `\check`
    Check,
    /// `\tilde`
    Tilde,
    /// `\acute`
    Acute,
    /// `\grave`
    Grave,
    /// `\dot`
    Dot,
    /// `\ddot`
    Ddot,
    /// `\breve`
    Breve,
    /// `\bar`
    Bar,
    /// `\vec`
    Vec,
    /// `\dddot` — amsmath builds it from three dot marks in a row; so does
    /// the layout here (no bundled face carries U+20DB).
    Dddot,
    /// `\ddddot` — four dot marks, same construction.
    Ddddot,
    /// `\mathring`
    Ring,
    /// `\widehat`
    WideHat,
    /// `\widetilde`
    WideTilde,
    /// `\overline`
    OverLine,
    /// `\underline` (math mode; in text mode `\underline` is a
    /// [`TextStyle::Underline`] island)
    UnderLine,
    /// `\overbrace` (annotations attach as scripts on the wrapping
    /// [`NodeKind::Scripts`] node, exactly as TeX attaches them)
    OverBrace,
    /// `\underbrace`
    UnderBrace,
    /// `\overrightarrow`
    OverRightArrow,
    /// `\overleftarrow`
    OverLeftArrow,
}

impl AccentKind {
    /// True for accents that sit above the base (everything except the
    /// under-class accents).
    #[must_use]
    pub const fn is_over(self) -> bool {
        !matches!(self, Self::UnderLine | Self::UnderBrace)
    }
}

/// The text-mode styling islands of the TexText contract.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum TextStyle {
    /// `\textbf{…}`
    Bold,
    /// `\emph{…}`
    Emph,
    /// `\underline{…}` in text mode
    Underline,
}

/// The argument-taking math alphabet commands.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum MathFont {
    /// `\mathbb` (and `\mathds`, which the default preamble pack maps here)
    Blackboard,
    /// `\mathcal`
    Calligraphic,
    /// `\mathrm`
    Roman,
    /// `\mathbf`
    Bold,
    /// `\boldsymbol`
    BoldItalic,
    /// `\mathsf`
    SansSerif,
    /// `\mathtt`
    Typewriter,
    /// `\mathit`
    Italic,
}

/// The explicit spacing commands.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum SpaceKind {
    /// `\,` — 3 mu
    Thin,
    /// `\:` — 4 mu
    Med,
    /// `\;` — 5 mu
    Thick,
    /// `\!` — −3 mu
    NegThin,
    /// `\quad` — 18 mu (1 em)
    Quad,
    /// `\qquad` — 36 mu (2 em)
    Qquad,
    /// `\ ` (control space) — an ordinary interword space
    ControlSpace,
}

impl SpaceKind {
    /// The width in mu (18 mu = 1 em at the current size). The control
    /// space is nominally a text interword space; 6 mu (= ⅓ em) is the
    /// conventional math approximation.
    #[must_use]
    pub const fn mu(self) -> i32 {
        match self {
            Self::Thin => 3,
            Self::Med => 4,
            Self::Thick => 5,
            Self::NegThin => -3,
            Self::Quad => 18,
            Self::Qquad => 36,
            Self::ControlSpace => 6,
        }
    }
}

/// The phantom flavors.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum PhantomKind {
    /// `\phantom` — occupies width, height, and depth.
    Full,
    /// `\hphantom` — occupies width only.
    Horizontal,
    /// `\vphantom` — occupies height and depth only.
    Vertical,
}

/// The `\stackrel`/`\overset`/`\underset` family.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum StackKind {
    /// `\stackrel{top}{base}` — the result is a Rel atom.
    Stackrel,
    /// `\overset{top}{base}` — the result takes the base's class.
    Overset,
    /// `\underset{bottom}{base}` — the result takes the base's class.
    Underset,
}

/// What a node is. See the module docs; every variant is produced by
/// [`crate::parse`] / [`crate::parse_text`] with full span provenance.
#[derive(Clone, Debug, PartialEq)]
pub enum NodeKind {
    /// A horizontal list: a group's content, a cell, an argument, or the
    /// whole formula.
    List(Vec<Node>),
    /// A single character atom, already mapped to its math codepoint
    /// (`-` ⇒ `−`, `*` ⇒ `∗`, `\pi` ⇒ `π`). `class` is the intrinsic atom
    /// class before contextual Bin→Ord degradation.
    Symbol {
        /// The (mapped) character.
        ch: char,
        /// Intrinsic atom class.
        class: AtomClass,
    },
    /// A big operator (`\sum`, `\int`, …): an Op atom with a limits mode.
    BigOp {
        /// The operator character (`∑`, `∫`, …).
        ch: char,
        /// `\limits`/`\nolimits` state.
        limits: Limits,
        /// True for the `\int` class, whose default is side scripts even
        /// in display style.
        integral: bool,
    },
    /// A roman operator name (`\sin`, `\lim`, `\operatorname{…}`): an Op
    /// atom set in upright text.
    OpName {
        /// The rendered name ("sin", "lim", …).
        name: String,
        /// True for the `\lim` class, which takes under/over scripts in
        /// display style.
        limits: bool,
    },
    /// Sub/superscripts and primes attached to a base atom. `base` is
    /// `None` when the script opens the list (TeX's empty-nucleus atom).
    Scripts {
        /// The atom the scripts attach to.
        base: Option<Box<Node>>,
        /// Subscript.
        sub: Option<Box<Node>>,
        /// Superscript (primes precede it visually).
        sup: Option<Box<Node>>,
        /// The `'` primes, one source span each.
        primes: Vec<Span>,
    },
    /// A generalized fraction: `\frac`-family, `\binom`/`\choose`, or an
    /// infix `\over` that split its enclosing list.
    Frac {
        /// Numerator.
        num: Box<Node>,
        /// Denominator.
        den: Box<Node>,
        /// Bar/delimiter/style flavor.
        spec: FracSpec,
    },
    /// `\sqrt`, with an optional index (`\sqrt[3]{x}`).
    Radical {
        /// The index, if any.
        index: Option<Box<Node>>,
        /// The radicand.
        radicand: Box<Node>,
    },
    /// An accented atom.
    Accent {
        /// Which accent.
        accent: AccentKind,
        /// The base.
        base: Box<Node>,
    },
    /// `\left … \right`: an Inner atom.
    LeftRight {
        /// Opening delimiter.
        left: Delim,
        /// Closing delimiter.
        right: Delim,
        /// The enclosed list.
        body: Vec<Node>,
    },
    /// A fixed-size delimiter (`\big(`, `\Big\{`, …).
    SizedDelim {
        /// Which size.
        size: DelimSize,
        /// The atom class the variant imposes: `\bigl` ⇒ Open, `\bigr` ⇒
        /// Close, `\bigm` ⇒ Rel, plain `\big` ⇒ Ord.
        class: AtomClass,
        /// The delimiter.
        delim: Delim,
    },
    /// `\text{…}` inside mathematics: the body is text-mode content.
    Text {
        /// Text-mode body.
        body: Vec<Node>,
    },
    /// A literal run of text-mode characters, with one source span per
    /// character (decoded characters and source bytes are not linear:
    /// escapes decode two bytes to one character, whitespace collapses) —
    /// the provenance `Text[3:7]`-style slicing consumes.
    TextRun {
        /// The decoded text.
        text: String,
        /// One span per `char` of `text`, in order.
        char_spans: Vec<Span>,
    },
    /// A `\textbf`/`\emph`/`\underline` styling island (text mode, or the
    /// LaTeX text-in-math form).
    TextStyled {
        /// Which style.
        style: TextStyle,
        /// The body, in text mode.
        body: Vec<Node>,
    },
    /// `$…$` (or `$$…$$`) inside text mode: the body is math-mode
    /// content.
    MathIsland {
        /// Math-mode body.
        body: Vec<Node>,
        /// True for `$$…$$` display mathematics (lays out in display
        /// style); false for inline `$…$` (text style).
        display: bool,
    },
    /// A style-switch marker (`\displaystyle` …) applying to the remainder
    /// of the enclosing list.
    StyleChange(Style),
    /// A line-alignment declaration (`\centering`): applies to the
    /// remainder of the enclosing list, exiting at group end, exactly like
    /// LaTeX's paragraph declarations. Produces no glyphs of its own.
    AlignChange(LineAlign),
    /// A size declaration (`\small`, `\Large`, …): sets the current size
    /// factor for the remainder of the enclosing list, exiting at group
    /// end, and composing multiplicatively with the script styles (the
    /// LaTeX 10 pt ladder of `size10.clo`).
    SizeChange(f64),
    /// A `\color{…}` marker applying to the remainder of the enclosing
    /// group. The argument is kept verbatim.
    ColorChange(String),
    /// A line-spacing declaration (`\doublespacing`): multiplies the
    /// \baselineskip of subsequent `\\`-stacked lines in the enclosing
    /// multi-line list (setspace's \baselinestretch, 10 pt ladder value).
    /// Produces no glyphs; inert inside grids, whose row rule is the
    /// environment's own.
    LineSpacing(f64),
    /// A math-alphabet command applied to one argument.
    MathFont {
        /// Which alphabet.
        font: MathFont,
        /// The argument.
        body: Box<Node>,
    },
    /// A phantom box.
    Phantom {
        /// Which dimensions it occupies.
        kind: PhantomKind,
        /// The hidden body.
        body: Box<Node>,
    },
    /// `\stackrel`/`\overset`/`\underset`.
    Stack {
        /// Which flavor.
        kind: StackKind,
        /// The small stacked element (top for stackrel/overset, bottom for
        /// underset).
        annotation: Box<Node>,
        /// The base.
        base: Box<Node>,
    },
    /// `\xrightarrow[below]{above}` / `\xmapsto[below]{above}`: a drawn
    /// arrow stretched to its script-style labels, spaced as a relation.
    XArrow {
        /// True for `\xmapsto` (the origin-bar band).
        mapsto: bool,
        /// The mandatory above-label (a `List` node).
        above: Box<Node>,
        /// The optional below-label (a `List` node).
        below: Option<Box<Node>>,
    },
    /// An explicit spacing command.
    Space(SpaceKind),
    /// `~` — a tie (non-breaking interword space).
    Tie,
    /// `\\` — a line break (or, at an environment's own level, the row
    /// separator, in which case it is consumed by the environment).
    Linebreak,
    /// `&` — an alignment tab (or, at an environment's own level, the cell
    /// separator, in which case it is consumed by the environment). Kept as
    /// a node at the top level because the Tex surface wraps whole strings
    /// in an `align*`-class environment.
    AlignTab,
    /// A line-alignment environment (`flushleft`, `center`, `flushright`):
    /// a text-mode block whose `\\`-split lines are aligned within the
    /// widest line's width. Lines are [`NodeKind::List`] nodes.
    AlignBlock {
        /// The line alignment.
        align: LineAlign,
        /// The lines, in order.
        lines: Vec<Node>,
    },
    /// A `\begin{name} … \end{name}` environment. Cells are
    /// [`NodeKind::List`] nodes.
    Environment {
        /// Environment name (`array`, `cases`, …).
        name: String,
        /// The column-spec argument (`array` only), kept verbatim.
        spec: Option<String>,
        /// Rows of cells.
        rows: Vec<Vec<Node>>,
    },
    /// A structural fragment: the Tex surface's multi-argument idiom makes
    /// each literal argument its own corpus string, so a piece may be a
    /// *substring of a balanced whole* (`"{a"`, `"b}"`, `"\right)"`). The
    /// grammar accepts these at the top level and marks them explicitly —
    /// never silently.
    Fragment(FragmentKind),
}

/// How the lines of a multi-line block are aligned horizontally within
/// the block's width (the widest line's width).
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
pub enum LineAlign {
    /// `flushleft` (and the default): every line flush left.
    #[default]
    Left,
    /// `center` / `\centering`: every line centered.
    Center,
    /// `flushright`: every line flush right.
    Right,
}

impl LineAlign {
    /// The fraction of a line's slack placed to its left: `0` flush left,
    /// `1/2` centered, `1` flush right.
    #[must_use]
    pub fn slack_factor(self) -> f64 {
        match self {
            Self::Left => 0.0,
            Self::Center => 0.5,
            Self::Right => 1.0,
        }
    }
}

/// The structural fragments the top level tolerates (per-argument
/// `SingleStringTex` semantics).
#[derive(Clone, Debug, PartialEq)]
pub enum FragmentKind {
    /// An unmatched `}` whose opener lives in an earlier piece. Transparent
    /// to classification and spacing; renders nothing.
    UnmatchedClose,
    /// A `\right` whose `\left` lives in an earlier piece; renders its
    /// delimiter (class Close).
    StrayRight(Delim),
    /// A redundant `$` in a math-mode string (authors wrapping an
    /// already-math string in dollars). Transparent; renders nothing.
    RedundantMathShift,
}